首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
Foreland-propagating external thrust belts may be considered as essentially plane strain phenomena so that displacements can be correlated throughout their linked, three-dimensional fault geometry. This approach has been applied to part of the northwest external French-Swiss Alps, around the Mont Blanc basement massif. Imbricates of basement and cover sequences on the SW margin of this massif restore to a width in excess of 77 km with an implicit shortening of at least 67 km. These displacements can be correlated with those in the neighbouring Helvetic nappes by transferring movements, via lateral branch lines, onto the Mont Blanc thrust. By reappraising thrust geometries, the Helvetic/Ultrahelvetic nappe complex has been restored to a width of 114 km to the ESE of the Aiguilles Rouges basement massif. Displacements on the internal (SE) margin of the Mont Blanc massif, estimated by balanced sections and a restoration of the Ultrahelvetic klippen in the sub-alps, exceed 59 km. Thrust continuity, incorporating the restorations of nappes and imbricate geometries around the Mont Blanc massif, is illustrated on a crude, restored branch-line map which also serves as a preliminary palaeogeographic reconstruction. External thrust systems, to the east of the external Belledonne/Aiguilles Rouges massif, restore to a width of at least 140 km in the footwall to the Frontal Pennine thrust.  相似文献   

2.
The Mont Blanc massif is one of a chain of basement culminations which crop out along the external French Alps. Its southwestern margin is interpreted as being a major thrust belt which propagated in a piggy-back sequence towards the foreland. These imbricates have developed in the footwall of the high-level Valais thrust. The depth to the floor thrust and shortening within imbricates above this thrust are estimated by a series of partially balanced cross-sections drawn between the ‘synclinal median’ and the Valais thrust. These sections restore to a pre-thrust length of at least 50 km, probably exceeding 100 km, above a floor thrust never deeper than 1 km below the sub-Triassic unconformity. All this thrust displacement is transferred via a series of lateral branch lines onto the Mont Blanc thrust in the Chamonix area. A corollary of this is that the Aiguilles Rouges and the main part of the Mont Blanc massif were separated by probably as much as 100 km prior to Alpine thrusting. Such large shortening estimates imply a hitherto unsuspected Dauphinois stratigraphic consistency in both thickness and lithology.To achieve a balance a restored crustal cross-section must show an equal length of both lower and upper crust. Thus a high-level basal detachment which floors large thrust displacements must overlie a long, undeformed lower crustal wedge. A restored section 100 km long requires such a lower crustal wedge to exist beneath the entire Alpine internal zones. Perrier & Vialon's crustal velocity profile through the western Alps is reinterpreted in these terms. The Ivrea body is considered to be a portion of an external lower crustal wedge which has been uplifted by thrusts after most of the displacement on the external thrust belt.  相似文献   

3.
The tectonic evolution of the Mont Blanc range with regard to its cooling and exhumation history has been discussed and debated over many years and is still controversial. Recently, several low-temperature thermochronology studies have determined the cooling history of the massif in considerable detail and various tectonic models proposed to explain the young and fast exhumation signal established from these studies. Here we present detailed field data from the wider Mont Blanc area and assess possible exhumation processes in terms of these field constraints. Our observations indicate that none of the major faults or shear zones around the Mont Blanc massif (i.e. Mont Blanc shear zone, Mont Blanc back-thrust, Penninic thrust) was active in Late Neogene times and that young exhumation is therefore not controlled by movements along these structures. We demonstrate that the position of Mont Blanc in the bend of the western Alps plays an important role in its tectonic history and that simple 2D models are insufficient to explain its evolution. Interference between NW–SE compression and orogen-parallel extension along the Rhône-Simplon fault system resulted in a complex regional structural pattern, with strike-slip movements on both sides of the Mont Blanc massif. Young brittle faults are predominantly strike slip without significant vertical offset. The young (<2 Ma) rapid exhumation of Mont Blanc is more broadly distributed and cannot be directly linked to discrete faults bounding the massif. The mechanisms driving this recent accelerated exhumation must similarly be of broader scale.  相似文献   

4.
Abstract

Three generations of mylonites occur in the Mont Blanc Massif :

— the first one has produced mylonitic schists trending N20°E, of regional extension; the late Variscan Mont Blanc granite intruded these mylonites;

— the second one has affected large areas in the Mont Blanc granite, with the same N20°E direction;

— the third one is located along rather narrow bands N45°E, in the granite as well as in the older mylonitic schists.

All theses mylonites have been generated under (Bi) - greenschist facies conditions. They exhibit steeply plunging schistosity and lineation. They are all ante-Alpine; paradoxically, Alpine structures within the massif do not exhibit a striking appearence.  相似文献   

5.
Field evidence around the southwestern termination of the Mont Blanc basement massif casts doubt on Butler's interpretation of the massif as a relatively thin thrust-sheet extending over the imbricated Dauphind cover. A thick-skinned model of basement faulting, analogous to Laramide faulting in the western U.S.A., seems more appropriate for the area.  相似文献   

6.
The northeastern portion of the Mont Blanc massif in western Switzerland is predominantly comprised of the granitic rocks of the Mont Blanc intrusive suite and the Mont Blanc basement gneisses. Within these metamorphic rocks are a variety of sub-economic Fe skarns. The mineral assemblages and fluid inclusions from these rocks have been used to derive age, pressure, temperature and fluid composition constraints for two Variscan events. Metamorphic hornblendes within the assemblages from the basement amphibolites and iron skarns have been dated using 40Ar/39Ar, and indicate that these metamorphic events have a minimum age of approximately 334 Ma. Garnet-hornblende-plagioclase thermobarometry and stable isotope data obtained from the basement amphibolites are consistent with metamorphic temperatures in the range 515 to 580 °C, and pressures ranging from 5 to 8 kbar. Garnet-hornblende-magnetite thermobarometry and fluid inclusion studies indicate that the iron skarns formed at slightly lower temperatures, ranging from 400 to 500 °C in the presence of saline fluids at formational pressures similar to those experienced by the basement amphibolites. Late Paleozoic minimum uplift rates and geothermal gradients calculated using these data and the presence of Ladinien ichnofossils are on the order of 0.32 mm/year and 20 °C/km respectively. These uplift rates and geothermal gradients differ from those obtained from the neighbouring Aiguilles Rouges massif and indicate that these two massifs experienced different metamorphic conditions during the Carboniferous and Permian periods. During the early to late Carboniferous period the relative depths of the two massifs were reversed with the Aiguilles Rouges being initially unroofed at a much greater rate than the Mont Blanc, but experiencing relatively slower uplift rates near the termination of the Variscan orogeny. Received: 23 April 1996 / Accepted: 27 August 1996  相似文献   

7.
Rockfalls and rock avalanches are a recurrent process in high mountain areas like the Mont Blanc massif. These processes are surveyed due to the hazard they present for infrastructure and alpinists. While rockfalls and rock avalanches have been documented for the last 150 years, we know very little about their frequency since the Last Glacial Maximum (LGM). In order to improve our understanding, it is imperative to date them on a longer timescale. A pilot campaign using Terrestrial Cosmogenic Nuclide (TCN) dating of five samples was carried out in 2006 at the Aiguille du Midi (3842 m a.s.l.). In 2011, a larger scale study (20 samples) was carried out in five other test sites in the Mont Blanc massif. This paper presents the exposure ages of the 2011 TCN study as well as the updated exposure ages of the 2006 study using newer TCN dating parameters. Most of these exposure ages lie within the Holocene but three ages are Pleistocene (59.87?±?6.10 ka for the oldest). A comparison of these ages with air temperature and glacier cover proxies explored the possible relationship between the most active rockfall periods and the warmest periods of the Holocene: two clusters of exposure ages have been detected, corresponding to the Middle Holocene (8.2–4.2 ka) and the Roman Warm Period (c. 2 ka) climate periods. Some recent rockfalls have also been dated (<?0.56 ka).  相似文献   

8.
Summary A new Swiss gold occurrence at Mont Chemin, comprising gold-bearing quartz veins, displays many characteristics that are typical of mesothermal gold deposits within the Alps and globally. The most notable of these features are: i) the presence of NaCl-H2O-CO2-bearing fluid with an XCO2 of approximately 0.016 and NaCl equivalents in the range 4.6 to 10.6 weight percent, ii) greenschist formational temperatures and pressures in the range 265-285 °C and 700-1400 bars; and iii) the proximity of the occurrence to the Rhone-Simplon Line, a deep crustal structure in the Swiss Alps.Corrected Ar-Ar data for hydrothermal adularia, considered to be contemporaneous with mineral deposition from the gold-bearing fluid, yields an age of 9.9 ±1.0 Ma. Geothermal gradients and uplift rates derived from the Ar-Ar age data and the geothermometry are in agreement with existing data for this region, and indicate that the hydrothermal activity at the Mont Chemin gold occurrence records one of the last Alpine metamorphic events in the northeastern Mont Blanc massif.Temperature estimates from fluid-muscovite-quartz-feldspar equilibrium and oxygen isotope thermometry of coexisting adularia and quartz are combined with the fluid inclusion isochores to derive depositional pressures. These data yield geothermal gradients on the order of 50 °C/km and uplift rates of 0.44 mm/a for the NE portion of the Mont Blanc massif.
Evidenz aus Flüssigkeitseinschluß-, stabilen Isotopen- und Ar- Ar Daten fü r das Alter und den Ursprung Gold führender Quarzgänge am Mont Chemin, Schweiz
Zusammenfassung Ein neues Schweizer Goldvorkommen am Mont Chernin, es handelt sich um Goldführende Quarzgänge, zeigt viele Charakteristika, die für mesothermale Goldlagerstätten der Alpen und weltweit typisch sind: i) Die Anwesenheit von NaCl-H2O-CO2 Fluiden mit einem XCO2 von ca. 0.016 und NaCl zwischen 4.6 und 10.6 Gew. % Äquiv.ii) Grünschieferfazielle Bildungstemperaturen und -drucke von 265-285°C bzw. 7001400bar. iii) Die Nähe der Vorkommen zur Rhone-Simplon Linie, einer tiefgreifenden Struktur in der Kruste der Schweizer Alpen.Korrigierte Ar-Ar Daten von hydrothermalem Adular, der als zeitgleich mit den Minerallagerstätten gebildet, angesehen wird, ergaben ein Alter von 9.9 ± 1.0 Ma. Die aus aus den Ar-Ar Daten bestimmten geothermalen Gradienten und Hebungsraten und die Ergebnisse der Geothermometrie stimmen mit bisher existierenden Daten aus dieser Region überein und zeigen, daß die hydrothermale Aktivität in den Goldvorkommen des Monte Chemin eines der letzten alpidischen metamorphen Ereignisse im nordöstlichen Mont Blanc Massiv darstellt.Temperaturabschätzungen aus Fluid-Muscovit-Quarz-Feldspat Gleichgewichten und Sauerstoffisotopen-Thermometrie an koexistierendem Quarz und Adular werden mit den Isochoren der Flüssigkeitseinschlüsse kombiniert, um die Bildungsdrucke abzuleiten. Diese Daten ergeben geothermische Gradienten in der Größenordnung von ca. 50 °C/km und Hebungsraten von 0.44 mm/Jahr für den Nordostteil des Mont Blanc Massives.
  相似文献   

9.
  相似文献   

10.
Apatite fission track dating from a central transect in the Argentera massif (southernmost External Crystalline Massif = ECM) yielded ages between 8.05 ± 0.6 and 2.4 ± 0.2 Myr, with a positive age/altitude correlation above 3 Ma, 1200 m. Recognising a thermal peak at c . 250°C, 33 Ma, based on stratigraphic, metamorphic and 39Ar/40Ar data, the present results suggest a slow cooling rate (8–5°C) for the Argentera massif during the Oligocene–early Pliocene. This rate compares with that from the Pelvoux massif, but contrasts with those observed in the northern ECM (Mont-Blanc and Aar: up to 14°C Myr−1) for the same time interval. This can be related to the different location of the ECM within the collided European margin. At about 3–4 Ma, the denudation rate would have increased up to c . 1 mm yr−1 in the Argentera massif, reaching the same value as in the Belledonne and northern ECM, likely a consequence of Penninic thrust inversion.  相似文献   

11.
In the Belledonne massif, the steep Paleozoic Belledonne Middle Fault (BMF) separates micaschists, displaying numerous landslides, from amphibolites. The massif is incised by the lower Romanche river valley. When crossing the BMF, the valley widens into a lozenge-shaped basin recently interpreted as an active pull-apart type structure associated with a major N110 striking Quaternary fault. Multidisciplinary investigations were carried out in the basin to check if this model has implications on the seismic and landslide hazard assessment. This study demonstrated the existence of a N80 sinistral strike slip Séchilienne Fault Zone (SFZ). This fault zone is suspected to offset the BMF by 375 m across the basin. Geophysical experiments revealed that the bedrock depth increases strongly in the basin, up to 350 m. Our study invalidates the active pull-apart origin of the basin and suggests it results from Quaternary glacial and fluvial erosion processes, magnified by the intersection of two inherited structures, the BMF and the SFZ.  相似文献   

12.
 The use of hydrothermal simulation models to improve the prediction of water inflows in underground works during drilling is tested in the Mont Blanc tunnel, French and Italian Alps. The negative thermal anomaly that was observed during the drilling of this tunnel in 1960 is reproduced by long-term, transient hydrothermal simulations. Sensitivity analysis shows the great inertia of thermal phenomena at the massif scale. At the time of tunnel drilling, the massif had not reached thermal equilibrium. Therefore, a set of simulation scenarios, beginning at the end of the last glacial period, was designed to explain the anomaly encountered in the tunnel in 1960. The continuous cooling of alpine massifs due to infiltration of waters from the surface has occurred for 12,000 years and is expected to continue for about 100,000 years. Comparisons of water-discharge rates simulated in the tunnel with those observed indicate that this hydrothermal method is a useful tool for predicting water inflows in underground works. Received, May 1998 · Revised, March 1999 · Accepted, April 1999  相似文献   

13.
Late-glacial ice advances in the western Italian Alps   总被引：1，自引：0，他引：1  
The extent of ice on the south side of the Mont Blanc massif during two late-glacial readvances is known primarily from the distribution of crystalline erratics on sedimentary and metasedimentary terrain, and from the distribution of end moraines and outwash remants. Similar dual moraine complexes are found elsewhere in upper Val d'Aosta and indicate that the pattern is regional in character. During the earlier readvance equilibrium-line altitudes were ca. 450 ± 50m lower than at present, whereas during the later readvance they were ca. 350 ± 50 m lower. The moraines, not yet closely dated, were deposited sometime after 14,200 14C Years ago by which time Lago'd Alice near the margin of the würm moraine system at Ivrea had been deglaciated, and before 8400 years ago, at which time Rutor Glacier in upper Val d'Aosta was no larger than at present. The younger moraines are believed to correlate with Egesen moraines on the north side of the Alps, whereas the older set may either correlate with an early phase of the Egesen or with Daun advance.  相似文献   

14.
The tectonics of the Chenoua massif suggests block rotations of Neogene nappes associated with the African–European plate convergence. To estimate the extent of these rotations, a Paleomagnetic study on rhyolites and andesites of Langhian–Serravallian age and sandstones of Burdigalian age was carried out on 23 sites (200 specimens). The sites are distributed in the northwestern, southeastern and southern Chenoua massif. One or two components of magnetization, mainly carried by magnetite, pyrrhotite and/or hematite, were isolated in sandstones and volcanics. The sandstone sites reveal magnetizations in sandstones from the Cap Blanc syncline that are post-folding. However, both polarities are found, which is consistent with data from Africa during the Upper Miocene. Clockwise and counterclockwise rotations were recorded, dating back to the Neogene times in volcanics and sediments. From the faulted Cap Blanc syncline counterclockwise rotations of 1?±?4° to 18?±?28° around a vertical axis occurred in sediments since the Miocene with respect to Africa. In fact, remagnetizations occurred at several periods of time and in different sites, providing information on the evolution of post-tectonic rotations. Some volcanics record counterclockwise rotations of about 30° since the Miocene, whereas others do not show any significant rotation. This can be explained by the direction of the principal compressive stress axis σ 1 and by lateral extrusions related to an indentation model, in which we expect both clockwise and counterclockwise rotations.  相似文献   

15.
Spatial control for in situ dating of mineral phases in fine‐grained rocks is a significant challenge in geochronology, and the precision of microsampling is a crucial factor in obtaining accurate results. In this study, a new microsampling approach to Rb–Sr geochronology has been applied to greenschist facies mylonitic shear zones in the Mont Blanc‐Aiguille Rouges region of the western European Alps. Using a laser‐ablation system for microsampling by laser cutting followed by conventional TIMS Rb–Sr isotopic analysis of μg‐sized samples provides an improved workflow for texturally controlled, quasi in situ dating of mineral phases. The automated cutting process minimizes material loss and the risk of handling errors, while facilitating sampling of complex shapes of almost any size, a significant improvement over earlier microscope‐mounted microdrills. The new Rb–Sr white mica–calcite ages of between 27 and 30 Ma indicate Oligocene deformation in Alpine shear zones from two specific areas in the Mont Blanc‐Aiguilles Rouges region.  相似文献   

16.
Dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes. This objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ U‐Th‐Pb geochronology of allanite. A greenschist facies shear zone within the Mont Blanc Massif is documented. Allanite is synkinematic and belongs to the mylonitic assemblage. LA‐ICP‐MS U‐Th‐Pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common‐Pb component. The use of measured Pb‐isotope compositions of associated minerals (feldspars and chlorite) is critical for accurate common‐Pb correction, and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage. A mean 208Pb/232Th age of 29.44 ± 0.95 Ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions. This age reflects the timing of the Mont Blanc underthrusting below the Penninic Front and highlights the potential of directly dating deformation with allanite.  相似文献   

17.
The different segments of the tectonic boundary between external (European) and internal (Penninic) units in the Western Alps, the so-called Penninic Front (PF), formed at different times and according to different kinematic scenarios. During a first episode (Eocene), the PF corresponds to a transpressive suture zone between Penninic and European units. North- to NNW-trending stretching lineations, found along internal nappe contacts within the Penninic units, are related to this episode. This subduction zone was sealed by the Priabonian flysch of the Aiguilles d'Arves, a detrital trench formation that formed during the final stages of subduction. During a second episode, starting in mid-Oligocene times, the PF, imaged along the ECORS-CROP profile, acted as a WNW-directed thrust. This thrust, the Roselend Thrust (RT), only partially coincides with the PF. South of Moûtiers, the RT propagates into the Dauphinois units, carrying the former Eocene PF (including the Priabonian flysch) passively in its hangingwall. South of the Pelvoux massif the RT finds its continuation along the "Briançonnais Front", an out-of-sequence thrust behind the Embrunais-Ubaye nappes. On a larger scale, our findings indicate oblique (sinistral) collision within the future Western Alps during the Eocene, followed by westward indentation of the Adriatic block.  相似文献   

18.
ABSTRACT The pressure-temperature and temperature-time paths derived for rocks in the Kohistan arc and adjacent Nanga Parbat-Haramosh massif record the dynamics of the collision between the island arc and the Indian plate. Studies of P-T-t paths show that the Kohistan arc was thrust over the Nanga Parbat-Haramosh massif at least 25 Ma ago, but not more than 30–35 Ma ago. Rocks in the Kohistan arc followed decreasing pressure paths, with the early metamorphism beginning at high pressures (9.5 kbar) and later metamorphism occurring at 8.0 kbar. In contrast, rocks in the Nanga Parbat-Haramosh massif (Indian plate) experienced increasing pressure and temperature paths. Prior to thrusting, the massif was at low pressures (4.0 kbar) and low temperatures (450°c). Later, the pressure and temperature increased to 8 kbar and 580°c. The authors interpret the convergence (to approximately the same pressure and temperature) of the P-T paths in the two terranes as being the result of thrusting and thermal equilibration between the thrust sheets. 40Ar/39Ar cooling ages of hornblendes and other geochronological data suggest that the time of peak metamorphism and hence the completion of thickening was approximately 30–35 Ma ago. Temperature-time paths show that after thrusting, during the period 25–10 Ma, the Kohistan arc and Nanga Parbat-Haramosh massif were uplifted at similar rates (0.5 km Ma). However, in the past 10 Ma the Nanga Parbat-Haramosh massif has been uplifted more rapidly than the adjacent Kohistan arc. Rapid uplift has been accommodated by late faults along the edge of the massif.  相似文献   

19.
《International Geology Review》2012,54(10):1276-1294
The North Dabashan thrust belt, which is located in South Qinling, is bounded by the Ankang fault on the north and the Chengkou–Fangxian fault on the south. The North Dabashan thrust belt experienced multiple stages of structural deformation that were controlled by three palaeostress fields. The first structural event (Middle Triassic) involved NNW–SSE shortening and resulted in the formation of numerous dextral strike-slip structures along the entire Chengkou–Fangxian fault zone and within the North Dabashan thrust belt, which suggests that the South China Block moved to the NW and was obliquely subducted under the North China Block. The second structural event (Late Triassic–Early Jurassic) involved NE–SW shortening that formed NW–SE-trending structures in the North Dabashan thrust belt. The third structural event (Late Jurassic–Early Cretaceous) involved ENE–WSW or nearly E–W shortening and resulted in additional thrusting of the North Dabashan thrust belt to the WSW and formation of the WSW-convex Chengkou–Fangxian fault zone, which has an oroclinal shape. Owing to the pinning of the Hannan massif and Shennongjia massif culminations, numerous sinistral strike-slip structures developed along the eastern Chengkou–Fangxian fault zone and were superimposed over the early dextral strike-slip structures.  相似文献   

20.
THE GROWTH MODE OF ALTUN FAULT AND IT'S DYNAMICS  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号